The present invention relates to a vehicle speed control device for automatically maintaining a constant vehicle speed.
An example of a conventional device of such type is disclosed in Japanese patent application Laid-open No. 39311/1983 which is shown schematically in FIG. 8 of this application. In FIG. 8, when a main switch 5 is closed by an operator of a vehicle and a battery 4 is connected to a control device 6 which includes a processing circuit 6a such as a microcomputer, the latter starts to process an output of a speed sensor 3 which comprises four magnetic poles, a rotary member 3a adapted to be rotated by a meter cable (not shown) by which a rotation of a transmission (not shown) is transmitted and a read switch 3b. The speed sensor 3 produces a pulse signal having a frequency proportional to a vehicle speed v.sub.s and the control device 6 measures a period of this pulse signal to obtain the vehicle speed v.sub.s. Then, when the operator closes a set switch 1 for giving an instruction of commencement of constant speed running, a signal is given to the control device 6 in which the speed v.sub.s at the time when the set switch 1 is closed is stored as a desired speed v.sub.r and constant speed control is started. Thereafter, the control device 6 compares the desired speed v.sub.r with an actual speed v.sub.s which may from time to time to provide a control signal corresponding to a difference therebetween. Thus, a motor type throttle actuator 7 is provided in a suction passage 8 or the engine which is responsive to various control signals from the control device 6 to rotate a link 7a by a motor connected thereto by means of an electromagnetic clutch to thereby regulate an opening of a throttle valve 9 through a rod 7b. That is, the throttle valve 9 is controlled such that, when the actual speed v.sub.s is lower than the desired speed v.sub.r, its opening is increased to a predetermined value by a throttle opening signal and, when it is higher than the desired speed, the opening is decreased. Therefore, it is possible to run the vehicle at a constant speed without necessity of acceleration pedal operation by the operator.
When the operator operates a brake device during such constant speed running control, a cancelling switch 2 for removing the constant speed running control is actuated to provide a cancelling signal to the control device 6. Upon the cancelling signal, the control device 6 provides a signal immediately upon which the actuator 7 causes the electromagnetic clutch to be separated. Thus, the operator can select the running speed by his operation of the acceleration pedal.
In such a conventional speed control device, however, the speed control is performed without taking an inclination of the road and/or a vehicle weight into consideration. Therefore, when the vehicle is going uphill, for example, a high gear ratio may be maintained, or, in downhill movement, a low gear ratio may be maintained continuously.